1. Field of the Invention
The invention relates to processes for the manufacture of semiconductor devices and more particularly to processes relating to the ion implantation of dopant impurities into a semiconductor wafer.
2. Description of Prior Art and Background to the Invention
The predominantly used method for the incorporation of dopant impurities into semiconductor wafers for the formation of semiconductive devices such as field effect transistors(FETs) and bipolar junction transistors(BJTs) is ion implantation. A focused beam of ions is generated in an apparatus called an ion implanter. The beam is produced by ionizing molecules or atoms in a source gas and then electrostatically collimating, filtering to exclude unwanted ions, and focusing the beam onto the target wafer surface, The focused beam, which ideally has a uniform radial density and spot size, is scanned across the wafer surface in a raster pattern so that the entire useable surface of the wafer is momentarily exposed to the beam. The ions are driven into (implanted) into the wafer surface. The depth to which they are implanted is dependent upon their energy within the beam. The dosage implanted depends upon the ion density in the beam and the time of exposure of an element of wafer surface area to the beam.
The actual density of ions is not uniform across the cross section of the focused beam which forms the spot. Consequently the implanted dosage varies from the center of the line of travel of the scan to the edge. In addition, for a scan performed by a circular spot, the instantaneous exposure of the scan line is greater at the center than at the edge. Consequently, the dosage of the resultant implant is greater at the center of the scan line than at the edges. In practice, the raster is programmed to provide an overlap between adjacent scan lines to overcome radial dosage non-uniformity and to assure continuous coverage. Scanning the wafer alternately in one direction and then in a direction orthogonal to the first improves the doping uniformity. Subsequent thermal processing also adds a further smoothing of the dopant profile. However, a shadow of the scanning raster is still retained, manifesting doping non-uniformities within areas of the order of tens of microns. Non-uniform ion densities within the beam or an unstable ion beam caused by tool contamination and tuning degradation can create impurity concentration variations on a more local scale causing variation of characteristics between adjacent devices. The angle of incidence of the beam on the wafer is also critical because of the channeling effect which is related to the crystalline orientation of the wafer. When a large wafer is scanned by a single ion beam, the angle of incidence must be maintained constant by tilting the wafer.
The effects of variations in the dopant profile become significant when the planar dimensions of the semiconductive elements shrinks to below the sub-half micron level and device performance becomes more crucially dependent upon dopant uniformity. This is of particular importance for ion implantations which are used for threshold voltage adjustment of MOSFETs (Metal oxide semiconductor field effect transistors).
Because of the manner in which the wafer is scanned it would be expected that the non-uniformity of the dopant profile would exhibit a periodic behavior, related to the scanning pattern. However, what is observed is that the dopant profile is more complex and not as predictable as would be expected from mere periodic considerations. It therefore becomes imperative to devise a method, not only to monitor the dopant distribution for process control but also to enable a better understanding of the mechanisms which affect it. The understanding of dopant uniformity becomes more important as device scaling extends to 0.25 microns and beyond because fewer and fewer charges are involved in forming the conducting channel necessitating the use of very high energy and very low beam current ion implanters.
Current methods for monitoring or measuring the uniformity of ion implant dosage are limited to the measurement of sheet resistance, capacitance voltage measurements and thermal wave measurements. These measurements are limited to determining average dosages over regions much larger than the dimensions of the scan lines patterns or of the implanted regions of the implant regions of small area devices, Non-uniformities in dopant depth and lateral concentration which occur because of improper scanning overlap, or channeling may extend over regions containing several devices. These non-uniformities result in significant variation in the performance of the devices. The current methods are incapable of measuring implanted wafer surfaces with fine enough resolution to reveal these dopant variations.
Pasch, et.al. U.S. Pat. No. 5,082,792 show a method for altering the resistance of a diffused region of a substrate through a contact hole in an insulating layer. The change in resistance is then used to determine the size of the hole. The invention does not address measurement of ion implanted impurity profiles in the wafer. Hsu, et.al, U.S. Pat. No. 5,451,529 shows a method for real time monitoring of ion implantation doses by measurement of the sheet resistance of a metal silicide monitor with a four point probe. The method does not have the resolution required to observe variations across ion implant beam scans. Ban Vavel, U.S. Pat. No. 5,432,352, Nakamura, et.al., U.S. Pat. No. 5,422,490, and Fujii, et.al., U.S. Pat. No. 5,574,280 show methods of ion beam control for ion implantation but do not show means for measuring implantation uniformity. Tabei, U.S. Pat. No. 4,570,070 show a method for monitoring the intensity of an ion beam by scanning a flat pellet of Al.sub.2 O.sub.3 with the ion beam and detecting the ultraviolet light emitted by the pellet.